JPH03264116A - Method for executing u-bending of metallic plate by press - Google Patents

Abstract

PURPOSE:To bend a metallic plate highly accurately into a U-shape by providing a prescribed width relief in the center of the width direction of a punch bottom and giving a limited % of compression strain in the bottom dead center of a press. CONSTITUTION:When a forming steel plate 1 is pressed by a punch into the groove of a die and bent in a U-sectional shape, a 1-3% compression strain is given at the press bottom dead center to both side end parts of the web of the metallic plate 1 from each of both side ends except parts having a bent angle R each on both sides of the web in the section to each of the insides within a quarter of the web width. In this way, shape variation after the steel plate is released from the die minimized and high-accuracy U-bending can be executed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a press zero bending method for metal plates, and is applicable to relatively thin-walled metal plates such as automobile bumpers.
The present invention also relates to a metal plate press zero bending method for forming a product made by bending a high-strength metal plate into a U-shaped cross-sectional shape with good dimensional accuracy.

[Conventional technology]

As is well known, in order to press and bend a relatively thin metal plate into a U-shaped cross-sectional shape, a groove die is used.
■There is a method of forming by repeating bending, and a method of bending and forming in one step using a grooved die.■When using a grooved die, there are restrictions on application depending on the web width and flange dimensions. , and from being inferior in terms of efficiency,
Generally, in the molding of products having a U-shaped cross-section, such as automobile bumpers, a method of bending the product in one step using a grooved die is often adopted.

In the press zero bending process using this mold groove die, as shown in FIGS. d and bend it into a U-shaped cross section (Fig. 5 a).
), at the bottom dead center of the press, the tip surface of the punch P (hereinafter referred to as
(hereinafter referred to as the punch bottom) and the inner bottom surface of the die groove d of the die D (hereinafter referred to as the punch bottom)
Pressure is applied to the flange surface of the metal plate M (hereinafter referred to as a final push) using a bottom press plate (referred to as a bottom press plate), and the metal plate M is shaped into the shape of the tip of the punch P to form a predetermined U-shape. The cross-sectional shape of FIG. 5b) is then achieved by releasing the mold.

In addition, during this press zero bending process, as shown in FIG. 6a, the metal plate M becomes "sag" below the tip of the punch P during its forming, but this "sag" The punch and die (hereinafter collectively referred to as the mold) are made to conform to the punch and die (hereinafter, both are collectively referred to as the mold) by pressing decisively at the bottom dead center of the press, thereby converging into the mold shape.

However, when the metal plate that has been U-bended in this way is taken out of the mold, the shape of the metal plate is different from that of the mold, and as shown in Fig. Go (hereinafter abbreviated as SG) phenomenon or springback (hereinafter abbreviated as SB) phenomenon in which the shape changes outward from each other as shown in Fig. 7, and the predetermined dimensions and shape cannot be obtained. There is.

Further, these SG or SB phenomena are due to the existence of a residual stress difference between the front and back sides of the metal plate due to U-bending in the thickness direction of the metal plate, and are caused by the elastic recovery force thereof. Therefore, if the yield strength of the metal plate material to be formed increases,
Since its elastic recovery power also increases, its SG or 5
Bi) becomes larger, so it is difficult to press and bend a high-strength metal plate exceeding a certain level, making it difficult to ensure the dimensional accuracy of the molded product, and it is extremely difficult to carry out the process.

On the other hand, in order to ensure safety in the event of a collision without increasing the weight of automobile parts, there is a trend toward using high-strength copper plates to further increase the specific strength of the parts. For this reason, recently, 100 kgf/mm'' class high strength steel plates have been used, and furthermore, the use of ultra high strength steel plates of 120 to 140 kgf/mm'' class is being considered. There has been a desire for a method that can be used to precisely press zero-bend these high-strength and ultra-high-strength steel plates.

Therefore, element analysis and countermeasures for these SG or SB phenomena have been studied from various angles.
For example, according to pages 165-168 of "Press Molding Difficulty Handbook (Nikkan Kogyo, 1987)", these SG
The SB phenomenon is shown in FIG. 8, which is an explanatory diagram of its angle change elements. ”-A.

The size of the angle change element of each of the three bending elements AB and BC affects the direction and magnitude of the angle change at the flange end, and the OA element is the web part created during forming. The "sag" in the flange is bent back at the bottom, and this elastically recovers after demolding, resulting in the SG at the flange.
The acid component is produced as a negative angle change component), the A''-"B element is produced as 5BIti, minute (positive angle change component) by bending by the punch shoulder R, and the BC element is A" in the process of bottoming out. It has been pointed out that a portion between -B is bent back, resulting in an SG component (negative angle change component). Also,
As a countermeasure, dimensional accuracy can be improved by improving the precision of each of the above-mentioned angle changing elements and by balancing these elements. When the radius of both upper edge corner parts R (hereinafter abbreviated as mold groove shoulder R) becomes larger, the bending fulcrum becomes wider, and the bulge of the web becomes larger during the bending process, resulting in 〇-A and B-C when bottoming out. ■ A method for suppressing the occurrence of SB and improving accuracy by increasing the die groove shoulder R radius in cases where large SB occurs at the flange end due to a large negative angle change between Selection of the clearance between the punch and the die: In other words, how to improve accuracy by minimizing the clearance between the punch and the mold groove of the die, and by improving the staining of the metal plate against the mold, ■Punch tip Selection of the shape; that is, take a relief at the bottom of the punch,
In addition to this method, the residual strain difference between the front and back surfaces is alleviated by bending the "°sag" produced in the web in reverse at the time of bottoming out to generate a negative angle change element, thereby increasing accuracy. By making the radius of the corner R on both sides of the punch tip (hereinafter abbreviated as punch shoulder R) as small as possible and limiting the contact between the web surface and the bottom of the punch to the periphery of both sides of the bottom of the punch when hitting the bottom, ° Methods such as widening the range of reverse bending of ``sag'' to further enhance the effect have been adopted.

[Problem to be solved by the invention]

However, when the material strength increases or the yield strength of the material fluctuates, as in the case of the high-strength steel plate mentioned above, applying the above-mentioned conventional technology requires understanding the optimal forming conditions each time, and selecting the optimal tool. Unless the shape is changed to a molded product with good dimensional accuracy, it is not possible to obtain a molded product with good dimensional accuracy.In addition, when the material strength is extremely high like the ultra-high strength steel plate mentioned above, there is a limit to the shape control effect. However, it is extremely difficult in practice to mold these with high precision, and the current situation is that there is still no sufficient countermeasure.

In view of the above conventional problems, the present invention has a relatively thin wall,
Another object of the present invention is to provide a zero-pressure bending method for a metal plate that can suppress the occurrence of SG or SB and form a high-strength metal plate with high precision.

[Means for Solving the Problems] In order to achieve the above object, the metal plate press zero bending method of the present invention presses the metal plate into the mold groove of the die with a punch to form a U-shape. When performing press zero bending into a cross-sectional shape of , a compressive plastic strain of 1 to 3% is applied to both ends of the web of the metal plate at the bottom dead center of the press.

[Effect]

In order to obtain guidelines for finding a method for accurately bending extremely high-strength metal plates such as ultra-high-strength Takatsuki with zero stress, the inventors of the present invention investigated the use of Hakkai-style materials in the U-bending process. Detailed deformation behavior was analyzed based on FEM (finite element method).

From the results of the investigation, it was found that among the elements 〇-A, A B-B C shown in Fig. 8, which have been pointed out previously, the material behavior between the OA of the web surface plays a major role in the angle change of the final product. Moreover, we found that the behavior is not simple.

Here, to explain the investigation procedure, the tensile strength is 30k.
Various steel plates of gf/mm2 class to 100 kgf/mm2 class were subjected to zero-pressure bending, and the influence of the angle change elements corresponding to each element OA, A B, B C shown in the above-mentioned Fig. 7 was investigated. However, in this investigation, after the final push at the bottom dead center of the punch P, the steel plate M to be formed is not released from the mold and the SB state is examined, but as shown in Fig. 2 a, which is an explanatory diagram. , After the decisive push, first, 5TE
At P 1, the upper part of both shoulders R of punch P (diagonal lines A and A in the figure)
At 5TEP 2, remove the restraints from both PR parts of the punch P (shaded areas B and B in the figure), and finally, at 5TEP 3, remove the restraints from the bottom of the punch P (shaded area C in the figure). part), and remove the restraint of the flange part at each of these 5 TEPs.
By measuring the B angle and analyzing these measured values, we investigated the influence of the flange (B C) near the R section, the R section (B C), and the bottom (OA) on the overall SB. It is.

The SB angle type values of the flange portion measured at each of these 5 TEPs are illustrated in the graph of FIG.

In addition, in the graph of Figure 2, the 7th mark is 5.
P30 (30kgf/mm” class) steel plate, ◎ mark o
7 shows the SB angle value of SP60 (60kgf/open 2nd class) net plate, and the plot marked with . is the SB angle value at each 5TEP of SPloo (100kgf/mm" class) steel plate. In addition, the SB angle value shown in the same graph Square angle type negative value is IsG angle θI
is synonymous with

According to the graph in Figure 2, the SB angle type of the flange part in each 5TEP changes greatly, especially the SP
This tendency is noticeable in the case of high-strength materials such as LOO (100 kgf/mm2 class).

Also, SB square angle type at each 5TEP, 5TEP 1,
All the values are negative, indicating the SG phenomenon, and increasing toward the SG side in proportion to the material strength.

In 5TEP 2, the SG recognized in 5TEP 1 returned to the same angle as the mold dimension, and at this stage, accuracy close to the mold dimension was obtained, and moreover, with high-strength steel. It can be seen that good accuracy can be obtained even if

Also, this means that the 5BF due to bending at the punch shoulder R part
The li, minute (positive angle change component) and the SG force (negative angle change component) of the flange near the R section are in a nearly balanced relationship, canceling out each angle change element. suggests that.

Furthermore, in STεP3 where the constraint on the bottom of the punch is removed,
The value is larger than the value for each SB angle type and 5TEP 1, and becomes a negative value again, and at this time also increases toward the SG side in proportion to the material strength.

As a result of further detailed analysis of these phenomena, as schematically shown in Figures 3a to 3C, the ``sag'' that occurs in the web portion of the broken steel sheet M during forming is caused by the punching. From the bottom punch stage lS (Figure 3a) just before the bottom dead center of P, as the punch P is pressed down, it receives a reaction force from the bottom push plate of the die D and is reversely bent (Figure 3b). However, at this time,
A large bending stress is generated between the reverse bending part of the web shown in Figure 3 and the bending R parts on both sides, that is, the lower part and the part of E2 in the figure, and a stress gradient is generated in the thickness direction. A certain plastic strain is introduced, and the stress difference between the front and back surfaces caused by the plastic strain in the lower and E parts remains even after the mold is flattened by pressing (Fig. 3C), resulting in mold release. It has become clear that the SG phenomenon appears in subsequent molded products.

That is, when looking at the molded product as a whole, it was found that both cases can be classified as an SG phenomenon caused by plastic strain in the lower and E parts of the web.

Furthermore, from the analysis results of the stress gradient distribution in the plate thickness direction in the vicinity of the bending R region, the regions of the lower and E parts of the web where plastic strain with a stress gradient is introduced vary depending on the web width, but the bending R As shown in FIG. 4, which is a schematic diagram of the stress gradient distribution in the thickness direction near the part, the point where the punch shoulder R transitions to the flat punch bottom in the cross section, that is, the straight line forming the punch bottom, and the point where the punch shoulder R transitions to the flat punch bottom, Punch shoulder R inscribed in a straight line
It has become clear that the distance from the intersection point P with the arc line forming the arc line toward the center of the web is approximately within 1/4 of the distance between the intersection points on both sides.

Based on these analysis results, the present inventors have determined that it is possible to eliminate or reduce the stress difference between the front and back surfaces due to plastic strain with a stress gradient that occurs in the lower and E parts of the web during press zero bending. , even for high-strength materials, it is now possible to create a highly accurate U-bent bottom shape, and
In order to eliminate or reduce this stress difference, it was concluded that it is effective to introduce compressive strain into the lower and E parts at the bottom dead center of the press.

Therefore, in order to clarify the validity of this conclusion and the amount of compressive strain that should be applied, we changed the pressing load applied to the lower and E parts of the flange at the bottom dead center of the press, and ! The relationship between (X) and the angle change of the flange of the molded product was investigated. The general purpose structural analysis program ABAQjiS was used for this analysis. In addition, as a break-through steel plate, 100kgf/mm with a plate thickness of 1.21 is used.
The results are shown in Table 1.

In addition, the material properties of the 100 kgf/m-edge high strength steel plate used in these investigations are Young's modulus ε = 21000 kgf/m
m'', Poisson's ratio sea: 0.3, and its stress/plastic strain characteristics are shown in Figure 5.In addition, as a mold,
A punch with a cross-sectional width of 100 mm at the tip and a radius of shoulder R of 5 mm, and a die having a rectangular cross-section with a depth of 30 mm and a groove with a radius of shoulder R of 5 mm were used. . The clearance between the punch and the die groove was set to 1.3 mm, which was the thickness of the plate to be formed + 0.1 mm. In addition, the coefficient of friction between the steel plate and the mold is μm 0.15, and the parts to which the pressing load is applied at the bottom dead center of the press are the lower and E parts of the web shown in Fig. 3, and the range thereof is as follows: 1 of the web width excluding the bending angle R parts on both sides from each side edge of the web excluding the bending angle R parts on both sides
/4, which is 22.5 mm inside.

As shown in Table 1, when appropriate plastic deformation strain is applied to both sides of the web by pressing at the bottom dead center of the press, and when no plastic deformation strain is applied (No.
In comparison with Example 1), it was possible to significantly reduce the amount of SG in the molded product, confirming the validity of the above conclusion. It should be noted that as the amount of strain applied increases, the angular change (SG) value of the flange decreases as if drawing an exponential curve, and this decreasing trend is observed when the amount of strain is around 1.0%. There was a sharp change in areas with less than that, and a gradual change in areas with more.

Here, in the present invention, the amount of plastic deformation strain imparted to both sides of the flange in the cross-sectional width direction is limited to 1 to 3%. This is because if the amount of plastic deformation strain applied is less than 1%, the stress difference between the front and back surfaces due to the plastic bending that occurs near the punch shoulder of the web surface due to the reverse bending of the center portion will remain, resulting in stable and sufficient accuracy. In addition, if a compressive plastic strain of 3% or more is applied, the dimensional accuracy of the molded product can be further improved, but the required pressing force at the bottom dead center becomes extremely large. Therefore, a large-capacity press is required, which limits its application.In addition, it is necessary to greatly increase the rigidity of the mold, which creates constraints on the selection of mold materials, and requires construction to supplement this. This is because it becomes complicated and causes a significant increase in mold production costs, which is not practical.

[Example] An example of the present invention will be explained below with reference to FIG. 1st
The figure is a sectional view for explaining the press U bending process of the steel plate of this example, and shows the state at the time of final pressing at the bottom dead center of the press.

In Fig. 1, (1) is a steel plate to be formed, and this steel plate (1) is pressed with a punch (2) and a die (3) at the bottom dead center of the press, which is the final stage of U-bending. ) mold groove (
3a) shows a cross-sectional state in which the center is pressed.

Here, in this example, the punch (2) and the die (
3) was attached to the upper and lower anvils of a normal press (not shown), and the mold groove (3a) of the die (3) had a normal rectangular cross-sectional shape. On the other hand, at the center of the bottom of the punch (2) in the width direction in the cross section, a clearance of 1/2 width of the cross-sectional width W excluding both shoulders R of the punch (2) is provided, and W/4 on both sides of this cross-sectional width W are It was made to protrude downward by a predetermined amount from the center part.

Then, at the bottom of the punch (2) formed in this way, the first
At the time of final pressing shown in the figure, a compressive plastic strain of 1 to 3% is applied to predetermined areas on both sides of the flange cross section of the steel plate (1) to be formed.

With the above configuration, 100kgf/mm2 to 140kgf
/mm2 grade steel plate was used as the plate to be formed (1), and when an automobile hanger was subjected to press zero bending, each of the obtained molded products had an extremely small amount of shape change (SB or SO) after release from the mold. All exhibited good dimensional accuracy, confirming the excellent effects of the present invention.

〔Effect of the invention〕

The press zero bending method for a metal plate according to the present invention can easily eliminate or alleviate the stress difference that occurs between the front and back of the flange portion during the bending process by pressing firmly at the bottom dead center of the press. This method exhibits excellent practical effects in that it enables highly accurate U-bending even for ultra-high-strength steel plates and the like, whose use has been limited in the past.

[Brief Description of the Drawings] Fig. 1 is an explanatory sectional view showing the state at the time of final pressing of press 0 bending according to the embodiment of the present invention, Fig. 2a is an angle change element in press U bending according to the present invention Figure 2 is an explanatory diagram of the investigation procedure for each 5T of the investigation of angle change elements in press zero bending related to the present invention.
Graphs showing the spring back angle θ of the flange part in EP, FIGS. 3a to 3C are schematic diagrams showing the behavior of the material in press zero bending process related to the present invention, and FIG. A schematic diagram showing the stress gradient distribution in the plate thickness direction near the bending R part of a formed steel plate, FIG. 5 is a graph showing the stress/plastic strain characteristics of the material used in the analysis related to the present invention, and FIGS. Figure 6 is a conceptual cross-sectional view of conventional press 0 bending process, Figure 7a is an explanatory diagram of the spring-go (SO) phenomenon that occurs in conventional press 0 bending process, and Figure 7 is a conventional press 0 bending process. Spring bank (SB) produced during bending
) An explanatory diagram of the phenomenon, FIG. 8 is an explanatory diagram of angle changing elements in U-bending processing related to the prior art. (1) - Steel plate to be formed, (2) - Punch, (3) -
Die, (3a) - mold groove.

Claims (1)

[Claims] When pressing a metal plate into the mold groove of a die with a punch and press U-bending it into a U-shaped cross-sectional shape, bending of both sides is performed from each side edge of the web in the cross section, excluding the bending angle R portion on both sides. A compressive plastic strain of 1 to 3% is applied at the bottom dead center of the press to both ends of the web of the metal plate up to the inner side within 1/4 of the web width excluding the corner R part. A press U-bending method for metal plates.